This invention relates to an electromagnetic wave absorber, hereafter to be called `wave absorber`, more particularly to a wave absorber which absorbs waves, for example, coming to the wall surface of a multi-storied building without reflecting them thereby to prevent generation of ghost on the television receiver, or which is used as the interior wall of anechoic chambers to improve wave interception efficiency.
If large-scaled constructions such as multi-storied buildings and warehouses are present in a propagation area of waves emitted, for example, from a television tower, television microwaves impinge upon the exterior wall of such buildings and reflected thereby. Accordingly, if a television wave is received near the large-scaled building, ghost is generated on the screen of the television receiver due to the time difference between the wave coming directly from the television tower and the delayed wave coming after it is reflected by the wall surface of the building, which phenomenon has given rise to an environmental problem of wave interference.
In order to cope with such ghost generation, a countermeasure is taken in some large-scaled buildings to apply a wave absorber, on the external wall surface of such buildings, which is adapted to absorb waves as much as possible without substantially reflecting them thereby. Conventional wave absorbers each consist of a ferrite tile directly bonded with an adhesive to a predetermined size of concrete plate or of a ferrite tile bonded to a concrete plate through mortar and a metal plate. Thus, if matching is achieved between the impedance of the wave absorber as viewed from the wave emitting direction and that of the free space when a television wave impinging on the wave absorber attached on the wall surface of the multi-storied building and the like, the television wave will not be reflected and the ghost phenomenon can be cleared.
In fact, however, it is extremely difficult to achieve matching between these impedance values in the conventional wave absorber, and the reflection attenuation achieved thereby is merely at the level of about 15 dB for the VHF range channels 1 to 3 and about 20 dB for the VHF range channels of 4 or more. Moreover, since the conventional wave absorber is of a multilayered structure comprising a ferrite tile, mortar, a metal plate and a concrete plate as described above, the total weight thereof will inevitably be increased, making it difficult to apply the wave absorber onto the wall surface of the building, disadvantageously. Further, it can be pointed out that the ferrite tiles constituting the wave absorber are liable to drop off due to the difference between the expansion coefficients of the respective materials, layer separation at the resin adhesive or cracking in the concrete wall to be caused by the swelling after water absorption. As a countermeasure for preventing such drop off of the tiles, it can be contemplated to reduce weight of the wave absorber. However, it is very difficult to achieve such weight reduction without lowering wave absorption characteristics and permanence thereof.
The present invention has been proposed in view of the above problems inherent in the conventional wave absorbers and for the purpose of overcoming them in a suitable manner, and it is an object of this invention to provide a wave absorber which not only has a high level of wave absorption and a relatively light weight but also can be manufactured easily.